Continuous spermatogenesis in a normal individual requires maintenance of a pool of spermatogonial stem cells. These cells, which are the precursors to all mature spermatozoa, must achieve a balance between differentiation and self-renewal. Our current knowledge that stem cells lie outside of the blood testis barrier, that maintenance of the stem cell pool requires signaling between somatic cells and germ cells, and that the balance between stem cell renewal and differentiation is regulated, suggest it might be feasible to develop contraceptive agents that reversibly block the initiation of spermatogenesis. We propose to increase the basic knowledge of spermatogonial stem cell renewal and differentiation by studying the luxoid mutation. The luxoid mutant arose spontaneously in 1950 and was placed in cryostorage in 1970. We have had this mutant revived from cryostorage and shown that it causes a depletion of spermatogonial stem cells. Luxoid appears to cause the same phenotype as a mutation in the glial-derived neurotrophic factor (Gdnf) gene, the only other mutation known to affect the balance between spermatogonial stem cell renewal and differentiation. We propose to clone the gene that is responsible for the luxoid phenotype, assess whether the gene is required in the somatic or germ cell compartment of the testis, and determine if luxoid functions in the same genetic pathway as Gdnf. Our immediate studies will expand our understanding of the genetic regulation of spermatogonial stem cell renewal. Knowledge gained from our studies will shed light on the feasibility of developing contraceptive strategies aimed at blocking the initiation of spermatogenesis.